1. Field of the Invention
This invention relates to an improved method for the processing of spent nuclear fuel, and, more specifically, this invention relates to an improved process for recycling spent nuclear fuel containing transuranic and zirconium nitrides [(TRU, Zr)N] and recovering metal values and nitrogen-15.
2. Background of the Invention
A mixture of plutonium and zirconium nitrides is being considered as a prospective fuel for Plutonium Burner reactors, and for accelerator driven reactors such as those for Accelerator-driven Transmutation of Waste (ATW). The purpose of using said fuel is to consume or transmute plutonium into less hazardous elements. The fuel is comprised of about 75% zirconium nitride and 25% plutonium nitride. The nitride is from nitrogen-15 (15N), which has only a 0.366% natural abundance, compared to its more abundant isotope, 14N.
Current United States policy is to store unprocessed spent reactor fuel in a geologic repository. Long-term uncertainties are hampering the acceptability and eventual licensing of a geologic repository for nuclear spent fuel in the U.S., and driving up its cost. The resistance among Yucca Mountain Range residents and others regarding the government""s plans to deposit radioactive material in the Yucca Mountain Repository is a case in point.
Instead of long term storage of untreated radioactive materials, preliminary treatment of spent nuclear fuel is being explored, including partial utilization of the fissile material contained in the spent fuel. Accordingly, there is an emphasis upon developing new technologies for reprocessing and reutilizing spent nuclear fuels.
A number of processes exist for the processing and recycling of nuclear fuels. These processes often involve aqueous solutions. Due to the presence of water, aqueous solutions are neutron moderators. In thermal reactors, the neutrons that cause fission are at a much lower energy than the energy level at which the same neutrons were born from fission. Through collisions between water nuclei and neutrons created by the spontaneous fission of plutonium, water in the aqueous solutions lowers the neutrons"" kinetic energies and readies these neutrons to cause fission. Accordingly, neutron moderation makes the critical mass of plutonium low for aqueous solutions. The low critical mass necessitates the use of very low plutonium concentrations and redundant safeguards to assure fission control. This results in low plutonium throughputs. Thus, aqueous solution processing and recycling of nuclear fuels is inefficient and not cost-effective.
At present, no complete process exists for the processing and treatment of nitride-based nuclear fuels.
The recovery of 15N from spent nuclear fuels is also very important. Nitrogen, as nitride, is a large fraction of many nuclear fuels. The driving force of a nuclear reactor using nitride fuels is the thermal neutron flux. Reactor core designs require that the fissionable material capture a large fraction of the neutrons. However, if neutrons are intercepted prior to their capture by fissionable material, such as by such as structural materials, fuel matrix, fuel cladding, and coolant, the neutrons are lost, and the reactor design is less efficient. Atmospheric nitrogen is made up of two isotopes, 14N, and 15N with respective abundances of 99.634% and 0.366%. Of the two isotopes, 14N is more likely to be intercepted by non-fissionable material than 15N because the neutron cross-section of 14N is 91,500 times greater than the neutron cross-section of 15N. Thus, 15N is required for nitride fuels to make a reactor""s design reasonably neutron-efficient. Currently, costly isotope separation process is employed to capture 15N from the atmosphere for fuel manufacture. However, since 15N is so expensive, efforts have been made to also recapture 15N from spent fuel.
A method has been developed for the recovery of transuranic metals from nitrides by electrolysis in LiCl-KCl melts. Osamu Shirai, Masatoshi lizuka, Takashi Iwai, Yasufumi Suzuki, and Yasuo Arai, xe2x80x9cRecovery of Neptunium by Electrolysis of NpN in LiCl-KCl Eutectic Melts,xe2x80x9d Journal of Nuclear Science and Technology, Vol. 37, No. 8, pp. 676-681 (August 2000).
U.S. Pat. No. 5,372,794 awarded to LeMaire, et al. on Dec. 13, 1994 discloses a process for separation of actinides from aqueous solutions.
U.S. Pat. No. 5,132,092 awarded to Musikas on Jul. 21, 1992 discloses an aqueous process for the extraction of uranium (VI) and plutonium (IV).
U.S. Pat. No. 5,085,834 awarded to LeMaire, et al. on Feb. 4, 1992 discloses an aqueous method for separating plutonium from uranium and from fission products.
U.S. Pat. No. 4,740,359 awarded to Hadi Ali, et al. on Apr. 26, 1988 discloses an organic-aqueous process for recovering uranium values.
U.S. Pat. No. 4,399,108 awarded to Krikorian et al. on Aug. 16, 1983 discloses a carbothermic reduction method for the recovery of actinides.
U.S. Pat. No. 4,297,174 awarded to Brambilla on Oct. 27, 1981 discloses a pyroelectrochemical process for reprocessing irradiated nuclear fuels. The process involves dissolving fuel to be reprocessed in a fused-salt bath.
U.S. Pat. No. 4,092,397 awarded to Brambilla, et al. on May 30, 1978 discloses a method for the pyrochemical separation of plutonium from irradiated nuclear fuels, by thermal decomposition in molten nitrates.
U.S. Pat. No. 3,981,960 awarded to Brambilla, et al. on Sep. 21, 1976 discloses a reprocessing method of ceramic nuclear fuel in low-melting nitrate molten salts.
Several of these patents teach less efficient aqueous separation processes. Also, none of the aforementioned patents or publications disclose a method for capture and recycling of 15N. Further, none of the aforementioned patents or publications anticipate or suggest direct electrochemical reduction of transuranic nitrides and zirconium nitride in melts of the respective transuranic and zirconium metal chlorides.
A need exists in the art for a method and device for isolating elements of nitride-containing nuclear fuel. The method should not require aqueous or nonaqueous separation techniques. The method and device should separate the nitrides of transuranics and zirconium. Also, the method and device should allow for the different reduction potentials of transuranic metal ions and zirconium ion, and for the capture of 15N.
An object of the present invention is to provide a process of efficiently processing and recycling spent nitridebased nuclear fuels that overcomes many of the disadvantages of the prior art.
Another object of the present invention is to provide a nonaqueous system for the recovery of metal values from spent nuclear fuel rods. A feature of the invention is that the spent fuel rods are placed in a metal anode basket and transuranic metals and/or zirconium metal are isolated from, but recovered simultaneously with, 15N by direct electrochemical reduction. An advantage is that costs are lowered due both to fewer steps in the recovery of the metal and nitrogen, and to higher throughputs of transuranic metals.
Still another object of the present invention is to provide a method for the recovery of transuranic metals and zirconium metal by electrolysis in a bath of chlorides of the same metals. A feature of the invention is that transuranic metal ions and zirconium ions emanating from the anode replace the ions in the melt which in turn discharge at the cathode, forming the metal. An advantage of the invention is that the reduced metal is collected directly from the cathode in concentrated form.
It is another object of the present invention to provide a method for recovery of 15N, bound as nitride ion in fuel rods. A feature of the invention is that 15N is carried by an inert carrier gas over liquid nitrogen traps. An advantage of the device is that a considerable savings in costs occurs due to the capture and recycling of the 15N.
Yet another object of the present invention is to provide a device and method for separating transuranic metals and zirconium metal from alkali, alkaline earth, rare earth, and noble metal fission products and from each other. A feature of the invention is that residual fission product metals present in the spent fuel rods accumulate in both the metal anode basket and the chloride bath as oxidized metal. An advantage of the invention is that transuranic metals and zirconium metal of high purity are produced, separately in a serial procedure at a corresponding cathode in electrical communication with the anode. That is, separation and collection of one metal occurs prior to the separation and collection of another metal, even though both metals are contained in the same starting material matrix and the entire matrix is processed at the beginning of the isolation/purification procedure.
Briefly, the invention provides a process for separating the constituents of spent nuclear fuel comprising a first nitride compound and a second nitride compound, the process comprising loading spent fuel in an anode basket; suspending the basket in a first molten electrolyte; placing a first cathode in the first molten electrolyte; applying a first voltage between the anode basket and the first cathode so as to oxidize the metal contained in the first nitride compound and electrolyte and allow the oxidized metal to transform to elemental metal at the cathode; removing the anode basket; placing the anode basket in a second molten electrolyte; placing a second cathode in the second molten electrolyte; and applying a second voltage between the anode basket and the second cathode so as to oxidize metal contained in the second nitride compound and second electrolyte and allow the oxidized metal to transform to elemental metal at the second cathode.
The invention also provides a device for recovering transuranic metal, zirconium metal, and nitrogen-15 (15N2) from a mixture of nitrides, the device comprising a means for placing transuranic and zirconium nitrides in an electrolyte; a means for reducing transuranic metal ions; a means for reducing zirconium metal ions; a means for oxidizing nitride ions: a means for collecting the nitrogen gas; and a means for collecting the transuranic and zirconium metal.